1. Field of the Invention
The present invention relates to an adjustment method for a position detection apparatus, an exposure apparatus, and a device fabrication method.
2. Description of the Related Art
A projection exposure apparatus which projects and transfers a circuit pattern formed on a reticle (mask) onto, for example, a wafer via a projection optical system has conventionally been employed to fabricate a semiconductor device using photolithography.
Along with the micropatterning of semiconductor devices, the projection exposure apparatus is required to transfer a reticle pattern onto a wafer by exposure with a higher resolving power. A minimum line width (resolution) that the projection exposure apparatus can transfer is proportional to the wavelength of exposure light and is inversely proportional to the numerical aperture (NA) of the projection optical system. In view of this, the wavelength of the exposure light is shortening and the NA of the projection optical system is increasing.
An exposure light source has currently shifted from a superhigh pressure mercury lamp (i-line (wavelength: about 365 nm)) to a KrF excimer laser (wavelength: about 248 nm) and an ArF excimer laser (wavelength: about 193 nm), and the practical application of even an F2 laser (wavelength: about 157 nm) is in progress. Moreover, the adoption of EUV (Extreme Ultra Violet) light with a wavelength of about 10 nm to 15 nm is expected.
There has also been proposed immersion exposure that aims at increasing the NA of the projection optical system by filling at least part of the space between the projection optical system and the wafer with a liquid (e.g., a liquid with a refractive index higher than 1). The immersion exposure improves the resolution by increasing the NA of the projection optical system on the wafer side.
Along with such an improvement in resolution, the projection exposure apparatus is also required to improve the overlay accuracy, that is, the accuracy of overlaying several patterns on the wafer. In general, the overlay accuracy must be about ⅕ the resolution. Along with the micropatterning of semiconductor devices, it is increasingly becoming important to improve the overlay accuracy. To obtain a desired overlay accuracy, it is necessary to align the reticle and wafer with high accuracy. For this purpose, the projection exposure apparatus includes a plurality of alignment detection systems (i.e., position detection apparatuses).
Wafer alignment detection systems are roughly classified into two types, that is, the off-axis detection system and the TTL-AA (Through the Lens Auto Alignment) detection system. The off-axis detection system detects an alignment mark on the wafer without using a projection optical system. The TTL-AA detection system detects an alignment mark on the wafer with the alignment wavelength of non-exposure light via a projection optical system.
In recent years, the semiconductor device production mode is shifting from low-variety, high-volume production to high-variety, low-volume production. Along with this trend, an alignment detection system which can minimize detection errors in wafer processes under various conditions (with regard to, e.g., the material, thickness, film thickness, and line width) is demanded. For example, when the alignment detection system includes a TIS (Tool Induced Shift), it generates detection errors even when it detects an alignment mark with a symmetrical stepped structure. Detection errors are generated due to aberrations (in particular, coma aberration due to decentering) which cause TISs and remain in the optical system of the alignment detection system, and the tilt (optical axis shift) of the optical axis of this optical system. To provide an alignment detection system which can minimize detection errors in wafer processes under various conditions, it is necessary to reduce coma aberration and an optical axis shift of the optical system of the alignment detection system.
Under the circumstances, an adjustment method for an alignment detection system has been proposed, which reduces an optical axis shift by moving an optical member of the alignment detection system (adjusting the optical center of gravity) so that an asymmetrical waveform obtained upon detecting an adjustment mark becomes symmetrical. See Japanese Patent Laid-Open No. 9-167738 for details of this technique.
However, the conventional adjustment method performs the adjustment so that coma aberration and an optical axis shift of the optical system of the alignment detection system are canceled in total, so it does not reduce coma aberration and an optical axis shift of the optical system of the alignment detection system (make them close to zero).
A waveform obtained upon detecting the adjustment mark becomes asymmetrical not only due to the influence of coma aberration but also due to the influence of an optical axis shift. In some cases, the influences of coma aberration and an optical axis shift merely cancel each other even when the optical axis shift is adjusted by moving the optical member of the alignment detection system, so the coma aberration and optical axis shift which cause detection errors, in fact, remain. Consequently, the alignment detection system adjusted by the conventional adjustment method can minimize detection errors in a wafer process under a certain condition, but cannot minimize those in wafer processes under various conditions. In other words, the detection accuracy (alignment accuracy) of the alignment detection system adjusted by the conventional adjustment method changes for each wafer process.